In an internal combustion engine having a turbocharger, when a throttle valve arranged on the downstream side of a compressor is closed during a transition period from turbocharged operation to deceleration operation, the turbocharge pressure on the downstream side of the compressor increases even more between the compressor and the throttle valve, and the flow rate of air passing through the compressor is reduced. For this reason, surging of the compressor occurs which air flows back from the downstream side to the upstream side of the compressor. This causes noise and vibration, or trouble such as adverse effects on parts of the compressor.
To prevent such surging, a conventional controller disclosed in International Patent Application Publication No. WO 2015/145942 has been known, for example. In the controller, the flow rate of air passing through a compressor (compressor flow rate) and a pressure ratio between upstream and downstream sides of the compressor (compressor pressure ratio) are detected, and an amount of change in the compressor flow rate is calculated. Additionally, threshold of the amount of change in the flow rate is calculated on the basis of the detected compressor flow rate and compressor pressure ratio. Then, when the calculated amount of change in the compressor flow rate exceeds the threshold, it is determined that a surge will occur, and an air bypass valve is opened to prevent surging.
As described above, surging of a compressor occurs when a closing operation of a throttle valve during deceleration from a turbocharged operation causes the turbocharge pressure on the downstream side of the compressor to increase even more between the compressor and the throttle valve, and reduces the compressor flow rate. However, reduction of the compressor flow rate occurs even when the opening of the throttle valve is unchanged, such as when an operation state of a waste gate valve provided on the exhaust side of the turbocharger is changed, or when the engine speed is controlled to the reduction side at the time of a gear change.
Meanwhile, in the conventional controller, occurrence of surging is determined on the basis of the compressor flow rate and the compressor pressure ratio. Hence, if the compressor flow rate drops in a state as mentioned above where the opening of the throttle valve is unchanged, the controller erroneously determines that surging will occur and opens the air bypass valve, even though surging is not likely to happen. As a result, unintended torque-down occurs, whereby drivability is impaired, and frequent actuation of the air bypass valve reduces life.
There is a need to solve such problems by providing a controller of an internal combustion engine that can accurately determine the likelihood of occurrence of surging, to thereby avoid unnecessary opening of an air bypass valve and suppress frequent actuation, while surely preventing occurrence of surging.